1. Field of the Invention
This invention relates to single and double axial flow turbines, and more particularly to means for cooling a turbine rotor having axially successive rows of blades which are normally subjectible to axially decreasing motive fluid pressure.
2. Description of the Prior Art
High temperature steam turbines often require cooling for the hot and highly stressed moving components to insure reliable operation at such high temperatures. Thus, many arrangements have previously been proposed for cooling the exposed, hot, rotating turbine components by use of coolant fluids.
A highly effective arrangement for accomplishing such cooling is that described and claimed in the Beldecos and Leyland U.S. Pat. No. 3,206,166, issued Sept. 14, 1965, and assigned to the assignee of the present invention. (See FIG. 6 in the present application for an illustration of this arrangement.) In accordance with the Beldecos and Leyland arrangement, an axial flow steam turbine is provided in which the control or first expansion stage includes an annular row of rotor blades carried by a disc portion of the rotatable rotor structure and cooperatively associated with a nozzle vane structure portion of a stationary structure. Sealing means are provided between the disc and nozzle vane structure for minimizing motive fluid leakage therebetween from the desired main motive fluid path as it exits the control stage nozzle vane structure. A second sealing means is also provided between the control stage rotor disc and its associated nozzle vane structure spacially separated from the first sealing means and together therewith defining a sealing volume for collecting motive fluid leakage which escaped past the first sealing means. Motive fluid which enters the sealing volume is transmitted therefrom to a downstream turbine stage through a plurality of conduits, parts of which are provided in the stationary structure. A major portion of the motive fluid which has been expanded through the control stage's rotor blades reverses flow direction, sweeps over the exterior of the stationary structure and its connected supply conduit, and enters the turbine's second nozzle vane structure. While a small portion of the remainder of the once-expanded motive fluid escapes axially downstream from the control stage, much of the remainder is pumped through a series of axially directed apertures in the control stage rotor disc. Such pumping action forces the once-expanded and thus cooled motive fluid through the disc into a space between the stationary structure and the rotor in an axial direction which is substantially opposed to that of the main motive fluid path through the control stage. The pumping action provides a sufficient rise in pressure to oppose hotter motive fluid leakage through the second sealing means and causes the relatively cooler, once-expanded motive fluid to blanket the rotor while traversing its way to the second nozzle vane structure.
While the above-described cooling arrangement is highly successful, it is to be noted that the main motive fluid flow path must be axially reversed in passing from the control stage to subsequent and lower pressure expansion stages. Flow reversal of the motive fluid leaving the control stage results in significant thermodynamic losses which adversely affect turbine efficiency. These losses include static pressure reductions and velocity dissipation of the motive fluid during its travel between the control stage and subsequent lower pressure expansion stages. It is therefore desirable to avoid such motive fluid flow reversal and its attendant thermodynamic losses while retaining its rotor cooling characteristics. Avoidance of such flow reversal is desirable for double as well as single axial flow turbines.